Hirschsprung disease (HSCR) is a complex genetic disorder whose primary clinical feature is the absence of intrinsic ganglia in the distal intestine. Defects of sympathetic and parasympathetic function have also been described in HSCR patients. Mutations in any one of several genes can give rise to the peripheral nervous system (PNS) deficits of this disorder. Incomplete penetrance and variable severity of PNS defects, exhibited by HSCR family members carrying identical gene mutations, suggests that genetic background modulates disease severity. Sox10dom mice display all the features of aganglionosis and extra-intestinal autonomic deficits described in HSCR patients. Sox10dom mice have been extremely valuable for defining the role of the Sox 10 transcription factor in development of neural crest (NC) that contributes to the PNS. Our analysis of Sox10dom mice in F1-hybrid and congenic strains demonstrates that genetic background impacts the severity of aganglionosis. The phenotypic variation we observe mimics that seen in human HSCR sibs, and suggests that modifier loci influence development of Sox10 derivatives in mouse and man. In the proposed experiments, we will test the hypothesis that multiple modifier loci contribute to the severity of autonomic defects in Sox10dom mutants. To systematically localize all genomic regions that modulate the severity of aganglionosis, we will perform a genome survey of Sox10dom intercross progeny. Selective genotyping of progeny at the phenotypic extremes will be performed to establish linkage to loci that influence HSCR severity. Our preliminary analyses in candidate gene approaches have identified a major modifier of enteric development, and suggest that additional modifiers contribute to the severity of NC defects. We will use extended pedigrees of Sox10dom mice for fine-mapping through intervals of candidate loci and modifiers detected by genome survey. Intervals that associate with severity of phenotype will be investigated in our pedigrees with genotyping panels, derived from mouse genome sequence that can be posted as a shared resource for the genetics community. To investigate the relevance of enteric NC modifiers to autonomic phenotypes outside the gut, we will perform a comparative analysis of sympathetic and parasympathetic function in Sox10dom congenic lines. These studies will contribute to our knowledge of the genetic basis for variation in development and disease of autonomic phenotypes in the PNS.